Oxygen barrier and mechanical properties at high humidity are often critical in materials design for food packaging. Transmission of oxygen through food packages is important reason for food degradation and has effect on the shelf-life of the products (A L Brody, et al., Journal of Food Science, 2008, 73, R107; and J Lagaron, et al., Materials Science and Technology, 2004, 20, 1). Traditional barrier layer has been aluminium in foil or metallised form (Y Leterrier, Progress in Materials Science, 2003, 48, 1). Layers of barrier polymers such as poly(vinylidene chloride) (PVDC), poly(ethylene vinyl alcohol) (EVOH), poly(vinyl alcohol) (PVOH) and polyamide (PA) have sufficient properties, but are made from petroleum resources (G Strupinsky et al., A Twenty-Year Retrospective on Plastics: Oxygen Barrier Packaging Materials, San Francisco, Calif., 1998). Therefore, materials from renewable resources would be preferable for packaging applications.
New bio-based materials have been explored to develop barrier films (K S Miller et al., Trends in Food Science & Technology, 1997, 8, 228; K Petersen, et al., Trends in Food Science & Technology, 1999, 10, 52; A Gandini, Macromolecules, 2008, 41, 9491; A Gandini, Green Chemistry, 2011, 13, 1061-1083; S C M Fernandes, et al., Green Chemistry, 2009, 11, 2023; and T Kohnke, et al., Green Chemistry, 2012, 14, 1864). An interesting alternative is hemicelluloses, especially the widely available wood hemicelluloses and they have been extensively studied as oxygen barrier films (N M L Hansen et al., Biomacromolecules, 2008, 9, 1493; J Hartman, et al., Biomacromolecules, 2006, 7, 1983). However, wide-spread commercial application is still missing due to low average molar mass and difficulty in extraction in pure form.
High molar mass xyloglucan from tamarind seed waste has good film-forming and mechanical properties and remarkable oxygen barrier performance, at least at low humidity conditions (J Kochumalayil, et al., Journal of Materials Chemistry, 2010, 20, 4321; J J Kochumalayil, et al., Biomacromolecules, 2012, 14, 84; and WO 2012/150904). The oxygen permeability of xyloglucan (XG) at 50% RH is 0.5-2 cm3 μm1 m−2 day−1 kPa−1 and is comparable to commercial barrier polymers such as PVOH or hemicelluloses derived from wood (Hansen, 2008). Mechanical properties and film forming characteristics of XG are more favourable than for wood hemicelluloses due to much higher molar mass.
Transmission of oxygen through a polymer film depends on many factors such as material structure, temperature, and humidity (Miller, 1997). The mechanism of O2 transmission through polymer below glass transition temperature (Tg) is poorly understood. However, it is widely accepted that oxygen solubility, thermodynamics of polymer mobility and free volume factors contribute to the permeability below Tg (M Klopffer, et al., Oil & Gas Science and Technology, 2001, 56, 223; H Fujita, Fortschritte der Hochpolymeren-Forschung, 1961, 1; and W W Brandt, The Journal of Physical Chemistry, 1959, 63, 1080). The high barrier performance of polar polymers such as PVOH, PVDC or polysaccharides arise from the combined factors of favourable chain packing, high cohesive energy density (defined as cohesive energy per unit volume) due to strong secondary interactions and low free volume (Lagaron, 2004). Strong intermolecular interaction and low oxygen solubility will limit O2 permeation.
The oxygen barrier properties of highly polar polymers with high water solubility (PVOH or wood hemicelluloses or xyloglucan) become reduced when the polymer is exposed to high humidity. In order to get water-soluble polymers show good barrier properties at high humidity, the moisture sensitivity should be reduced so that significant intermolecular interactions are preserved under these conditions. This is a seemingly impossible enigma since hydrophobic modification leads to loss of water solubility, which is a disadvantage in the context of water-based green processing concept. Furthermore, intermolecular interactions may be substantially reduced (J Comyn, Polymer permeability, Elsevier Applied Science, London, 1985).
In WO 2012/150904 the oxygen barrier properties of xyloglucan coatings at high humidity was enhanced by creating a nanocomposite with a layered sodium-montmorillonite (MMT).